How to Make a MegaStar

Astronomers are still largely in the dark when it comes to understanding how the most massive stars form, but they are now pursuing several new strategies to solve this enduring mystery.

Galaxies are the building blocks of the universe. To understand how galaxies have evolved over cosmic time, you have to understand their overall ecology; that is, the global process of star birth from interstellar gas and dust, through stellar evolution and on to stellar death, when material enriched in heavy elements is returned to the interstellar medium and begins the cycle anew.

But the birth of most massive stars and star clusters, which dominate and define the beautiful spiral structure of disc galaxies, is still poorly understood. Without that understanding we may struggle to fully grasp the origin of smaller stars like the Sun, as well as the origin of the elements that enable life.

Low-Mass vs High-Mass Stars
Over the past two decades, astronomers have pieced together a fairly good idea of how low-mass stars (i.e. those with masses less than a few times the Sun’s mass) form. With numerous low-mass stellar nurseries accessible within a few hundred light years of the Sun, astronomers have been able to closely observe individual stars (or binaries) being born.

There are always more details to be figured out, of course, but the basic picture seems secure. It starts out with a very cold (only 8–10°K above absolute zero), very slowly-rotating cloud of gas and dust whose own internal gravity has, after many millions of years, finally started to irreversibly condense the cloud. A central condensation forms, heating and spinning up as it condenses further until a flattened, spinning proto-stellar nebula takes shape. The excess spin is shed via powerful jets along the polar axes, and the protostar heats up further, driving off the excess gas. The remaining rubble in the disc around the protostar forms the planets of a new solar system.

But more massive stars (i.e. those with more than 10 times the Sun’s mass) lead a much more profligate and spectacular life. They consume their nuclear fuel so rapidly that they live for less than 1% as long as the Sun, often ending their lives in a titanic supernova explosion.

The birth of massive stars is equally spectacular and rapid. The copious quantity of ultraviolet radiation they put out once they have formed greatly disturbs their natal environment. This makes it difficult to do the detective work that would tell us about the conditions needed to form such stars.

Additionally, high-mass stars are rare compared with low-mass stars. This means we have to look much further – thousands of light years – to find enough massive stellar nurseries to study.

This combination of chaos, speed and distance has proven to be a real stumbling block to our understanding of massive star formation.

The significance of this gap in our knowledge has only increased in the past few years. Astronomers now have more evidence that most stars, even ones like our Sun, tend to form in large clusters.

This means that, contrary to what has been thought in the past, our Sun may not have formed in isolation. Hence, solving the mystery of massive cluster formation has become even more relevant to our own origins.

A Population Census
Enter the Galactic Census of High- and Medium-mass Protostars (CHaMP) in 2006. This large-scale survey has recently compiled a large, uniform database of massive star and star cluster-forming clouds as part of a detailed and systematic study that is determining the demographics of massive star formation.

CHaMP started with the molecular clouds where these clusters form. The densest parts of these clouds, where the star-forming action is, occupy only a very small fraction of the total volume of the giant molecular clouds in which they are embedded. This also means that such dense clumps are sparsely spread on the sky, and are hard to find unless we have some other indicator of their location.

To find a large sample, CHaMP needed a set of molecular “finder charts” – maps that would tell us where all of these clumps lie within a given part of the sky. This was a problem since, for most of the galactic plane, there are only fairly low-resolution maps of emissions from the cold molecules that make up giant molecular clouds (such as carbon monoxide, which is the brightest and easiest molecule to detect). But it would have taken many decades to map large areas of sky with a telescope that had a large enough antenna to give the sensitivity and angular resolution needed.

Fortunately, a Japanese telescope located in Chile, NANTEN, had already made maps of several molecules over a portion of the Milky Way at a better resolution than the all-sky surveys. The NANTEN telescope’s choice of species to map – 12CO, 13CO, C18O and HCO+ – turned out to be ideal for the creation of finder charts since each of these molecules successively traces denser gas than the prior one, and each was mapped in turn when the prior species was easily detected.

In this way, NANTEN “peeled the onion layers” of density in the structure of the molecular clouds, and did it in a relatively small amount of time. In Figure 1 the NANTEN maps of the CHaMP window show how we can zoom in on massive star formation sites this way, in this case going from 12CO to 13CO to C18O.

With these maps in hand, my NANTEN colleagues and I could go to the Mopra telescope near Coonabarabran in NSW and zoom in at much higher resolution on the dense clumps where massive stars were most likely to form.

This effort proved very successful. We found 300 dense molecular clouds with masses ranging from only 10 times the Sun's mass to as much as 20,000 times the Sun's mass. These clouds are the likely birthplaces of massive stars. Many of them already have clusters deeply embedded within them. They are masked optically by the huge amounts of dust present with the gas but viewable to ground- and space-based telescopes operating in the infrared.

We now have a treasure-trove of data on these clouds from Mopra, which we will be analysing for several years yet. But the sheer number of clouds we see means that many of them, while massive, are not actually forming massive stars for much of their existence. Instead, they are relatively “quiescent” and long-lived compared with how little time it takes for a truly massive cluster, once formed, to disperse the cloud.

We have submitted these results for publication to The Astrophysical Journal.

Global Collapse
A large survey like CHaMP also gives us another bonus: identifying unique or special objects that teach us something new by their rarity. My NANTEN and Anglo-Australian Observatory (AAO) colleagues and I published one such discovery last year in the Monthly Notices of the Royal Astronomical Society.

Among our 300 clouds we noticed only one that showed signs that it was collapsing in on itself due to gravity in a similar way to what is seen in low-mass protostar formation. Similar, yet different: this cloud, several light years across and with a mass equivalent to 20,000 suns or more, seems to be undergoing a global collapse at a rate that sends a Sun’s mass worth of material into its central regions every 30 years. At this rate, the entire cloud will be consumed in only half a million years, making perhaps hundreds of new stars.

In follow-up observations at the Australian Astronomical Telescope (AAT), just down the road from Mopra, the IRIS2 infrared camera has revealed some deeply embedded protostars at the centre of this collapse. These protostars are very luminous, very massive, and yet still forming. Figure 2 combines the AAT image with even longer wavelength infrared images from the orbiting Spitzer Space Telescope. We seem to be witnessing the birth pangs of a new cluster of massive stars.

The investigation continues. We have most recently obtained even higher-resolution images of these protostars at mid-infrared wavelengths from the Gemini South telescope in Chile, in which Australia is a partner, and were awarded time last October at the Australia Telescope’s Compact Array near Narrabri in NSW to obtain higher-resolution molecular maps as well.

The infall rate for this cloud is very interesting – it is both fast and slow. Since we selected a uniform population of clouds to map, the proportion of clouds showing a given trait gives us a direct indication of the relative length of time this trait lasts over the whole lifetime of the cloud. This is elementary demographics.

The fact that we see only one such object among 300 clouds studied means that the collapse phase must last less than 1% of the typical lifetime of the clouds. This makes the collapse “fast”. Half a million years for collapse means the clouds live for 50 million years or more. This is a much longer cloud lifetime than some astronomers prefer, based on other studies.

But the collapse is also “slow”. With gravity alone, a cloud this massive and of this size should collapse in just 30,000 years. This means that other physics besides gravity – such as the injection of turbulence into the cloud from protostellar jets buried deep within – is slowing the cloud’s evolution. This is a problem for some theories of star formation, but supports other theories.

Large-scale Surveys
CHaMP is no longer the only large-scale survey of molecular clouds and massive star formation in the Milky Way using Australian facilities. HOPS (begun in 2008 and led by Andrew Walsh at James Cook University) and MOGSS (begun in 2009 and led by Erik Muller, who is now at Nagoya University) are investigating other approaches to this mystery.

A meeting of the “millimetre community” – referring to the radio wavelengths typically used to study such clouds – was held in Sydney in late 2008, and a loose collaboration of astronomers was formed to further these aims at three different frequencies. The MALT90 project, which focuses on molecules with emission lines near 90 GHz, commenced last winter at Mopra. Led by Jim Jackson at Boston University, it is making small maps of 3000 clouds identified by Chile’s APEX telescope. Walsh’s MALT45 project will begin examining molecular lines around 45 GHz at the Australia Telescope Compact Array soon, and Muller and I began MALT110 at Mopra last November to completely map 60° of the southern Milky Way simultaneously in four more spectral lines near 110 GHz..

These surveys will have a large impact on the future of Southern Hemisphere astronomy. Besides the pioneering science they will enable, they will also provide a very large legacy data set for the multi-national Atacama Large Millimetre Array (ALMA) in Chile, which is beginning early science this year.

Between all these community-wide collaborations, the future for our understanding of massive star and star cluster formation in the Milky Way looks very bright indeed. We will soon be able to answer with more certainty the question: “How do you make a megastar?”

Peter Barnes is an Assistant Scientist in the Astronomy Department at the University of Florida.